The present invention relates generally to lithographic masks used for forming features on a substrate, and more particularly, to phase-shifting masks used for forming features on a semiconductor substrate.
Fabricating microelectronic devices typically includes forming features on selected layers of a semiconductor wafer. Individual features are often formed by using a mask to protect selected portions of a layer of material during subsequent processing steps. Masks may be used in the fabrication of virtually any type of microelectronic device, and are particularly useful in the fabrication of field emission displays (FEDs), which may be used in computers, television sets, camcorder viewfinders, and a wide variety of other applications.
FEDs are one type of flat panel display. In an FED, a baseplate with a generally planar emitter substrate is positioned relatively parallel to a faceplate having a substantially transparent display screen. The baseplate has a number of emitters formed on the emitter substrate that project from the substrate towards the faceplate. The emitters are commonly of a roughly conical shape, with the tips projecting towards the faceplate. An extraction grid having holes aligned with respective emitters is positioned between the emitter substrate and the baseplate. In operation, a potential difference is applied to the extraction grid and the emitters, thereby causing the emitters to emit electrons. The inner surface of the display screen is coated with a transparent conductive material and a cathodoluminescent layer. A potential difference is applied to the emitters and the conductive material to attract the electrons emitted by the emitters to the display screen. As these electrons pass through the cathodoluminescent layer to the conductive material, the cathodoluminescent layer emits light. For a general overview of FED technology, see D. A. Cathey, Jr., xe2x80x9cField Emission Displays,xe2x80x9d Information Display Vol. 11, No. 10, pages 16-20, October 1995, incorporated herein by reference.
Fabrication of the emitters can be done in accordance with a number of known processes. Typically, a plurality of regularly spaced areas are protected during anisotropic etching of the emitter substrate. These areas are protected by a patterned protective layer, such as silicon-dioxide overlying a silicon emitter substrate. Patterning the protective layer is accomplished by use of a mask, such as a photolithographic mask or reticle used to selectively expose a photoresist layer overlying the protective layer. Alternatively, a mechanical mask can be employed, such as beads or other relatively uniformly sized particles that are distributed over the protective layer.
Currently available masks used to fabricate FED emitters suffer from a number of problems. Mechanically distributed beads often cluster or are otherwise not uniformly distributed, thereby producing irregularly shaped and irregularly distributed emitters. Traditional photolithographic transmission masks selectively pass or block light to form respectively light and shadow regions on the photoresist layer. Improved resolution is achieved at the expense of reduced depth-of-focus and process throughput. Given the dimensions of the emitters (approximately one-by-one micrometer in cross-section at the substrate), traditional photolithographic techniques have significant depth-of-focus problems, thereby requiring the use of a highly planar (and correspondingly expensive) emitter substrate.
In accordance with one aspect of the present invention, a mask is provided for forming a pattern on a surface of a substrate. The mask has a substantially planar surface and is adapted to transmit light of a predetermined wavelength. The mask includes a field portion adapted to transmit the light and provide a first optical path length. The mask also includes a pattern portion adapted to transmit the light and provide a second optical path length. The first and second optical path lengths differ by approximately an odd integer multiple of one-half the predetermined wavelength. The pattern portion of the mask includes a plurality of pattern elements, each of which is substantially laterally surrounded by a respective part of the field portion. Each of the pattern elements has substantially equal spatial extent along first and second lateral directions that are substantially perpendicular to one another.
In accordance with another aspect of the present invention, a method is provided for fabricating a lithographic mask of a type for use in semiconductor device manufacturing. The mask is made from material that transmits light of a predetermined wavelength that is incident upon the mask in a first direction. The method of fabricating the mask includes covering a surface of the mask material with a protective layer. A portion of the protective layer is selectively removed to expose a corresponding portion of the mask material and leaving a remaining portion of the protective layer. Either the remaining protective layer or the exposed mask material includes a plurality of pattern elements, each of substantially the same size and separated from other pattern elements by the exposed mask material or the remaining protective layer, respectively. Each pattern element has substantially equal spatial extent along second and third directions that are substantially mutually perpendicular with each other and with the first direction. A portion of the exposed mask material is then removed, with the amount removed corresponding to an optical path length of approximately an odd integer multiple of one-half the predetermined wavelength. The remaining portion of the protective layer is subsequently removed.
In accordance with a further aspect of the present invention, a method is provided for fabricating a periodic structure in a substrate having a substantially planar surface, such as forming a plurality of field-emission display emitters in a semiconductor substrate. A protective layer is formed overlying the substrate. The protective layer is then substantially continuously exposed to light for no more than a single exposure time interval. The light undergoes substantially destructive interference at a plurality of regions of the protective layer. Each of these regions has substantially equal spatial extent along first and second lateral directions that are substantially perpendicular to one another. Portions of the protective layer are removed to expose underlying portions of the substrate, and portions of the exposed substrate are then removed.